Embodiments relate generally to piston-type marine vibrators for marine geophysical surveys. More particularly, embodiments relate to the restriction of gas flow in a piston-type marine vibrator to compensate for air-spring effects.
Sound sources are generally devices that generate acoustic energy. One use of sound sources is in marine seismic surveying in which the sound sources may be employed to generate acoustic energy that travels downwardly through water and into subsurface rock. After interacting with the subsurface rock, for example, at boundaries between different subsurface layers, some of the acoustic energy may be reflected back toward the water surface and detected by specialized sensors, in the water, typically either on the water bottom or towed on one or more streamers. The detected energy may be used to infer certain properties of the subsurface rock, such as structure, mineral composition and fluid content, thereby providing information useful in the recovery of hydrocarbons.
Most of the sound sources employed today in marine seismic surveying are of the impulsive type, in which efforts are made to generate as much energy as possible during as short a time span as possible. The most commonly used of these impulsive-type sources are air guns that typically utilize compressed air to generate a sound wave. Other examples of impulsive-type sources include explosives and weight-drop impulse sources. Another type of sound source that can be used in marine seismic surveying includes marine vibrators, such as hydraulically powered sources, electro-mechanical vibrators, electrical marine seismic vibrators, and sources employing piezoelectric or magnetostrictive material. Marine vibrators typically generate vibrations through a range of frequencies in a pattern known as a “sweep” or “chirp.”
Prior sound sources for use in marine seismic surveying have typically been designed for relatively high-frequency operation (e.g., above 10 Hz). However, it is well known that as sound waves travel through water and through subsurface geological structures, higher frequency sound waves may be attenuated more rapidly than lower frequency sound waves, and consequently, lower frequency sound waves can be transmitted over longer distances through water and geological structures than can higher frequency sound waves. Thus, efforts have been undertaken to develop sound sources that can operate at lower frequencies. Very low frequency sources (“VLFS”) have been developed that typically have at least one resonance frequency of about 10 Hz or lower. VLFS's are typically characterized by having a source size that is very small as compared to a wavelength of sound for the VLFS. The source size for a VLFS is typically much less than 1/10th of a wavelength and more typically on the order of 1/100th of a wavelength. For example, a source with a maximum dimension of 3 meters operating at 5 Hz is 1/100th of a wavelength in size.
In order to achieve a given level of output in the water, a marine vibrator typically needs to undergo a change in volume. In order to work at depth while minimizing structural weight, the marine vibrator may be pressure balanced with external hydrostatic pressure. As the internal gas (e.g., air) in the marine vibrator increases in pressure, the bulk modulus (or “stiffness”) of the internal gas also rises. Increasing the bulk modulus of the internal gas also increases the air-spring effect within the marine vibrator. As used herein, the term “air-spring” is defined as an enclosed volume of gas (e.g., air) that may absorb shock or fluctuations of load due to the ability of the enclosed volume of gas to resist compression and decompression. Increasing the stiffness of the gas in the enclosed volume, increases the air-spring effect and thus the ability of the enclosed volume of gas to resist compression and decompression. This increase in the air-spring effect of the internal gas tends to be a function of the operating depth of the source. Further, the stiffness of the acoustic components of the marine vibrator and the internal gas are the primary determining factors in the marine vibrator's resonance frequency. Accordingly, the resonance frequency generated by the marine vibrator may undesirably increase when the marine vibrator is towed at depth, especially in marine vibrators where the interior volume of the marine vibrator may be pressure balanced with the external hydrostatic pressure.